Human immunodeficiency virus (HIV) is the etiological agent of acquired immunodeficiency syndrome (AIDS). At present, there is no effective vaccine against this disease, and therapeutic agents provide only limited help. The objects of this project are to characterize HIV antigens, to determine the targets of humoral and cell-mediated immunity, and to use this information to develop candidate vaccines. We have constructed recombinant vaccinia viruses containing HIV genetic information. These viruses have been used as live experimental vaccines to immunize animals, to synthesize HIV proteins in tissue culture, to make targets for cytotoxic T cells, and to study CD4-envelope protein interactions. Our previous studies indicated that the native form of the HIV-1 envelope protein is a tetramer composed of two dimers. Mutagenesis studies demonstrated that amino acids 68 to 129 of gp41 are essential for oligomerization. The binding region appears to be quite broad, since point mutations within highly conserved amino acids were without effect. Monoclonal antibodies were produced that reacted preferentially or exclusively with oligomeric forms of the envelope protein. Expression of the HIV-1 gag-pol gene by a recombinant vaccinia virus was previously shown to result in the budding of retrovirus-like particles that contained active reverse transcriptase. Increasing the amount of the gag-pol protein caused premature activation of the protease and inhibition of particle formation. This assembly block could be reversed by treating the cells with a protease inhibitor or by mutagenizing the protease active domain.